Summary

Multiple vascular areas are formed in unincubated and prestreak chick blastoderms under the influence of multiple transplanted hypoblasts. Though the transplanted hypoblasts merge into one continuous layer, they do not participate collectively in forming one streak and embryonic axis with one area vasculosa at its postero-lateral end, but each hypoblast seems to form an embryonic centre or fraction of a centre. It may be that the merged hypoblasts do not lose their individuality or that they have induced their prospective embryonic centre before merging. This is an indication that the hypoblast is able to initiate certain events to which the epiblast responds. Composite blastoderms with stage-2 host epiblasts and with two or more transplanted hypoblasts form the embryonic axis at its prospective plane and two area vasculosae, the area opaca vasculosa (a.o.v.) as in the control blastoderm, and an induced area vasculosa 180° anteriorly. However, composite blastoderms with stage-3 epiblasts and with two or more transplanted hypoblasts behave as the control blastoderms forming the embryonic axis and the a.o.v. at their prospective sites. This indicates that the typical a.o.v. in the chick blastoderm is stabilized to blood island formation at stage 3. The stage dependence which involves progressive restriction of the areas in which blood islands will develop, suggests the existence of a centre which creates organization by integrating short-lived fields. It seems that there are no particular cell groups of the unincubated blastoderm determined to form erythrocytes but that the organizing capacities of the area are necessary to induce the first early commitments of prospective erythroblasts along this course.

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